1. Field of the Invention
The present invention relates in general to a fluid-filled active vibration damping device adapted to regulate the pressure of a non-compressible fluid in a fluid chamber formed therein, for applying an oscillating force to an object whose vibration is to be damped, in order to actively damp the vibration of the object. More particularly, this invention is concerned with such a fluid-filled active vibration damping device suitably used as an engine mount or a vibration damper for automotive vehicles.
2. Discussion of the Prior Art
For damping a vibration (including a noise due to the vibration) of an object to be damped, such as the body or other members of automotive vehicles which are subject to a vibration, there have been various vibration damping devices such as a vibration damping connector and a vibration damper. The vibration damping connector, such as an engine mount, is interposed between a vibration source and an object whose vibration is to be damped, in order to damp the vibration to be transmitted from the vibration source to the object. The vibration damper, such as a dynamic damper, is attached directly to the object to absorb or damp the vibration of the object.
To meet a need for improved vibration damping characteristics, there has been proposed a fluid-filled active vibration damping device, as an example of such a vibration damping device as described above, as disclosed in JP-U-61-191543, JP-B2-2510914 and JP-B2-2510915. Such a fluid-filled active vibration damping device includes an elastic body which partially defines a fluid chamber filled with a non-compressible fluid and which is elastically deformable by a vibrational load received from an object whose vibration is to be damped. The active vibration damping device further includes an oscillating body which also partially defines the fluid chamber and which is displaceable or deformable, and an actuator or drive means for oscillating the oscillating body. The oscillation of the oscillating body is controlled by applying to the drive means an electric signal corresponding to a characteristic of the vibration of the object, to thereby control the pressure of the fluid in the fluid chamber, so as to generate a controlled oscillating force to be applied to the oscillating body, so that the vibration of the object is actively or positively damped.
For such an active vibration damping device to be able to provide a sufficiently high vibration damping effect, it is desirable that the waveform of the oscillating force to be applied to the oscillating body, that is, the waveform of the fluid pressure change in the fluid chamber correspond to the characteristics of the vibration of the object to be damped. Where the vibration source is an internal combustion engine of an automotive vehicle, for instance, it is considered possible to apply an oscillating force to the oscillating body by energizing the drive means for generating an electromagnetic force or pneumatic force, with an electric current having a sine wave whose period, amplitude and phase correspond to those of the waveform of the vibration of the object, as disclosed in JP-A-8-72561 and JP-A-9-42374.
However, controlling the electric current so as to have a sine wave corresponding to the waveform of the vibration of the object requires an expensive control device with a complicated control arrangement, and tends to suffer from deterioration of the vibration damping effect due to a control delay associated with a high-frequency control of the drive means with an electric control signal. In particular, the drive means adapted to generate a pneumatic force by energization with a sine wave drive current is not necessarily capable of generating an effective oscillating force, since it is extremely difficult to regulate the pneumatic force whose magnitude varies along a sine wave, even if the control device can be arranged to generate the sine wave drive current as the electric control signal.
For solving the drawback indicated above, it is considered possible, for example, to use an electric pulse signal as the electric control signal for controlling the drive means. Where the active vibration damping device is used as an engine mount for an automotive vehicle, for instance, the electric control signal in the form of pulses whose frequency corresponds to that of the vibration of the object to be damped can be relatively easily obtained on the basis of a crank angle signal or an ignition timing signal of an engine of the vehicle. Further, the phase angle of the electric control pulse signal can be easily controlled by a central processing unit having a comparatively small processing capacity, and the magnitude of the generated oscillating force can be suitably adjusted by controlling the electric level control, duty ratio control or pulse width modulation (PWM) of the pulse signal.
An extensive study by the present inventors regarding the control of the drive means with an electric control pulse signal revealed that the use of the pulse signal for controlling the drive means is likely to cause undesired generation of higher harmonic components of the vibration frequency of the object, in the waveform of the fluid pressure change in the fluid chamber which takes place by oscillation of the oscillating body, and therefore in the waveform of the oscillating force to be applied to the object, not only where the drive means is adapted to generate a pneumatic force, but also where the drive means is adapted to generate an electromagnetic force or where the drive means uses a strictive element or elements. Thus, the use of the electric control signal in the form of pulses for energizing the drive means does not permit the active vibration damping device to exhibit a desired vibration damping effect, in the presence of the higher harmonics in the waveform of the oscillating force to be generated. Further, the use of the electric control pulse signal may even have a risk of deteriorating the vibrating condition of the object. Therefore, the use of the electric control pulse signal for the drive means still has a technical problem to be solved.